A fluorinated elastic copolymer is excellent in the heat resistance, the chemical resistance, the oil resistance, the weather resistance, etc., and accordingly applicable in a severe environment which a conventional hydrocarbon material cannot withstand. As a fluorinated elastic copolymer, a vinylidene fluoride/hexafluoropropylene copolymer, a tetrafluoroethylene/propylene copolymer, a tetrafluoroethylene/perfluoro(alkyl vinyl ether) copolymer, etc. have been known.
Such a fluorinated elastic copolymer is poor in reactivity and is thereby insufficient in crosslinkability and adhesion to other materials, and accordingly a method of introducing a reactive functional group thereby to improve the reactivity has been proposed. Particularly, in order to improve crosslinkability by a peroxide, a method of copolymerizing a special curable monomer or pre-treatment before crosslinking by a peroxide, has been carried out. For example, a fluorinated elastic copolymer obtained by copolymerizing vinylidene fluoride with hexafluoropropylene in the presence of a fluorinated chain transfer agent containing an iodine atom, contains an iodine atom at the terminal of the polymer, and accordingly peroxide crosslinking is possible (Patent Document 1).
On the other hand, a tetrafluoroethylene/propylene copolymer is excellent in the amine resistance and the high temperature steam resistance as compared with the above fluorinated elastic copolymer having repeating units of vinylidene fluoride. As such a tetrafluoroethylene/propylene copolymer, a fluorinated elastic copolymer obtainable by copolymerizing a monomer having a crosslinkable functional group such as a vinyl ester monomer has been proposed (Patent Document 2). However, such a tetrafluoroethylene/propylene copolymer is insufficient in flowability in a cavity of a mold in a case where a product having a complicated shape is to be produced.
A fluorinated elastic copolymer obtainable by copolymerizing a tetrafluoroethylene/propylene in the presence of a fluorinated chain transfer agent containing an iodine atom has also been proposed (Patent Document 3). However, by the method disclosed in the Patent Document, the polymerization rate is low, and the productivity of the fluorinated elastic copolymer is very low. Further, the fluorinated elastic copolymer to be obtained has insufficient crosslinkability, and physical properties of a crosslinked rubber such as the compression set are unsatisfactory.
Accordingly, development of a process for producing a fluorinated elastic copolymer excellent in the crosslinkability, having high flowability and excellent in the compression set and the base resistance has been desired.
Patent Document 1: JP-A-53-125491
Patent Document 2: JP-A-2006-70245
Patent Document 3: JP-A-5-222130